Asked how talks had gone with Mrs May, Jean-Claude Juncker shrugged his shoulders and spluttered “pfff”.
“We had no special event with Theresa May yesterday,” Mr Juncker said as he arrived at the summit.
“She was explaining what her intentions are. I’ll have lunch with her and then we will see what happens.”
No 10 sources said the one-to-one meeting with Mr Juncker later would be an “opportunity to establish a relationship” and talk about “how we see the process ahead”.
At the end of a dinner of pan-fried scallops, lamb with roast fig, and iced vanilla parfait, Mrs May set out Britain’s position on Brexit.
She told EU leaders Britain must not be shut out of discussions that affect all 28 members while it is still part of the bloc.
But the demands have caused anger in Brussels.
Manfred Weber, the leader of the European People’s Party bloc in the European Parliament, told BBC Radio 4’s Today programme: “When somebody wants to leave a club, it’s not really normal that such a member who wants to leave a club wants to decide about the future of this club. That is really creating a lot of anger, the behaviour of the British Government.
“It’s about the long-term project of the European Union, and the Brits decided not to stay in.”
The PM also told leaders directly that there was no going back on Brexit.
But after the dinner ended, Donald Tusk immediately told reporters he would be the “happiest one” if the UK reversed the decision to quit and stuck with the bloc for years to come.
The European Council president said: “It’s not our choice and if you ask me I would prefer 28 members not only for the next month, but also for the next years and decades.
“After the decision in the UK we have to respect the decision of the referendum. If it is reversible or not, this is in the British hands.
“I would be the happiest one if it is reversible but we now have to start our formal works.”
French president Francois Hollande has warned the UK would face “hard” negotiations if it wanted to make a clean break with the bloc, and European Parliament president Martin Schulz urged European leaders to “stand firm” in negotiations with Britain.
The UK’s former Brussels commissioner Lord Hill has cautioned against “stupid Brexit”.
He told Today: “We have this kind of false choice in the UK - often between hard Brexit and soft Brexit. I think the choice is between stupid Brexit and more intelligent Brexit, and that’s what we need to go for.”
Leaders are now discussing trade rules, including anti-dumping measures.